This invention defines a process and catalyst system, that facilitates the production of 1-octene in high selectivity, while avoiding the co-production of significant quantities of butenes, other octene isomers, specific higher oligomers and polyethylene. The catalyst system can also be used for the tetramerisation of other olefins, especially α-olefins.
Despite the well known value of 1-octene, the art does not teach a commercially successful process for the tetramerisation of ethylene to produce 1-octene selectively. Conventional ethylene oligomerisation technologies produce a range of α olefins following either a Schulz-Flory or Poisson product distribution. By definition, these mathematical distributions limit the mass % of the tetramer that can be formed and make a distribution of products. In this regard, it is known from the prior art (U.S. Pat. No. 6,184,428) that a nickel catalyst comprising a chelating ligand, preferably 2-diphenyl phosphino benzoic acid (DPPBA), a nickel compound, preferably NiCl2.6H2O, and a catalyst activator, preferably sodium tetraphenylborate, catalyse the oligomerisation of ethylene to yield a mixture of linear olefins. The selectivity towards linear C8 α-olefins is claimed to be 19%. Similarly the Shell Higher Olefins Process (SHOP process, U.S. Pat. Nos. 3,676,523 and 3,635,937) using a similar catalyst system is reported to typically yield 11 mass % 1-octene in its product mixture (Chem Systems PERP reports 90-1, 93-6 and 94/95S12).
Ziegler-type technologies based on trialkylaluminium catalysts, independently developed by Gulf Oil Chemicals Company (Chevron, e.g. DE patent 1,443,927) and Ethyl Corporation (BP/Amoco, e.g. U.S. Pat. No. 3,906,053), are also commercially used to oligomerise ethylene to mixtures of olefins that reportedly contain 13-25 mass % 1-octene (Chem Systems PERP reports 90-1, 93-6, and 94/95S12).
The prior art also teaches that chromium-based catalysts containing heteroatomic ligands with both phosphorus and nitrogen heteroatoms selectively catalyse the trimerisation of ethylene to 1-hexene. Examples of such heteroatomic ligands for ethylene trimerisation include bis(2-diethylphosphino-ethyl) amine (WO 03/053891, hereby fully incorporated herein by means of reference) as well as (o-methoxyphenyl)2PN(methyl)P(o-methoxyphenyl)2 (WO 02/04119, hereby fully incorporated herein by means of reference). Both these catalyst systems and processes are very specific for the production of 1-hexene and only yield 1-octene as an impurity (typically less than 3 mass % of the product mixture as disclosed by WO 02/04119). The coordinating phosphorus heteroatoms in (o-methoxyphenyl)2PN(methyl)P(o-methoxyphenyl)2 (WO 02/04119) are spaced apart by one nitrogen atom. It is believed that the nitrogen atom does not coordinate, at least in the absence of an activator, with the chromium and that without any further electron donating atoms on the ligand that it is a bidentate system. Furthermore it is argued that the polar, or electron donating substituents in the ortho-position of the phenyl groups help form a tridentate system, which is generally believed to enhance selectivity towards 1-hexene formation as reiterated by the inventor of WO 02/04119 in Chem. Commun., 2002, 858-859 by stating “This has led us to hypothesise that the potential for ortho-methoxy groups to act as pendent donors and increase the coordinative saturation of the chromium centre is an important factor.” To support their hypothesis, the authors of Chem. Commun., 2002, 858-859 showed that the use of (p-methoxyphenyl)2PN(methyl)P(p-methoxyphenyl)2, a compound without any such ortho-polar substituents on at least one of R1, R2, R3 and R4, as a ligand under catalytic conditions resulted in no catalytic activity towards α-olefins. WO 02/04119 (Example 16) teaches the production of octenes using a trimerisation of olefins process and catalyst system. In this instance, 1-butene was co-trimerised with two ethylene molecules to give 25% octenes. However, the nature of these octenes was not disclosed and the applicant believes that they consist of a mixture of linear and branched octenes.
The prior art teaches that high 1-octene selectivities cannot be achieved since expansion of the generally accepted seven-membered metallacycle reaction intermediate for ethylene trimerisation (Chem. Commun., 1989, 674) to a nine-membered metallacycle is unlikely to occur (Organometallics, 2003, 22, 2564; Angew. Chem. Int. Ed., 2003, 42 (7), 808). It is argued that the nine-membered ring is the least favoured medium sized ring and should thus be disfavoured relative to the seven-membered ring (Organometallics, 2003, 22, 2564). In addition, it is also stated by the same authors that, “if a nine-membered ring formed, it would be more likely to grow to an eleven- or thirteen-membered ring . . . . In other words, one would never expect much octene, but formation of some (linear) decene or dodecene would be more reasonable.”
Despite the teaching of the opposite, the applicant has now found a process for selectively producing a tetramerised olefin. The applicant has further found that chromium-based catalysts containing mixed heteroatomic ligands with both nitrogen and phosphorus heteroatoms, with polar substituents on the hydrocarbyl or heterohydrocarbyl groups on the phosphorous atoms, can be used to selectively tetramerise ethylene to 1-octene often in excess of 60 mass % selectivity. This high 1-octene selectivity cannot be achieved via conventional one-step ethylene oligomerisation or trimerisation technologies which at most yield 25 mass % 1-octene.